A radiation source often used for spectroscopic work in the infrared takes the form of a rod of suitable electrically resistive material which when heated by the passage of an electric current from end to end emits an infrared continuum in the spectral region for which the spectrophotometer is designed. The luminance of such a source is greatest around the mid-point thereof and tends to drop off at either side of it in a substantially symmetrical manner, as each of the two end terminals is approached.
The effect due to the thermal loading imposed by the terminals, can be minimized by having a comparatively long rod and using as the effective source a small medial segment away from the terminals. However, even at the limit of this approach that is acceptable in practice, the source is still marred by a central hot spot.
Wire-wound source elements in the references have very limited life due to oxidation of the metal wires at high temperature, which leads to failure when the wire burns out.
Large igniter elements typically require about 100 watts of power and, for spectroscopy applications, must be enclosed in a water cooled housing with a small aperture to limit the power that reaches the spectrometer components.
Small igniters, such as the Norton 301(now Saint-Gobain Advanced Ceramics, Inc., New Milford, N.H. 03055), typically require less than 20 watts, and can be used with an insulating housing that defines the aperture seen by the spectrometer optics, and typically do not require water cooling.
As illustrated in FIG. 1, prior art IR source 100 has an insulating housing 108 that surrounds lead wires 102 and 104, which are electrically coupled to, and supplied power from, a DC power supply or drive source 106 so that the lead wire 102 is a ground or negative lead while the lead wire 104 is a positive connection. The lead wires 102 and 104 are electrically coupled at junctions 122 and 124, respectively, to a resistance element that is formed of two support legs 112 and 114 and a radiating area or element 116 that is manufactured with a resistivity higher than that of the support legs 112 and 114. This configuration allows small power dissipation in the support legs 112 and 114 due to the drive current, and at the same time provides thermal insulation of the radiating element 116 from the support legs 112 and 114 that are at much lower temperature. The support leg 114 is therefore coupled to the positive (+) terminal of the DC power supply 106 while the support leg 112 is coupled to the negative (−) terminal of the DC power supply 106.
To provide a very stable output, the source 100 must be driven by a well-regulated current, so that the DC power supply 106 must be a precision power supply.
For optimal operation of the spectrometer, the optics that collect the source radiation are designed to focus only the high temperature spot of the source, otherwise the effective temperature illuminating the spectrometer field of view would be the average of the source temperatures in the total area observed.
The support legs 112 and 114 are built approximately symmetrical, and the hot spot 118, the highest temperature area, is centered between the two support legs 112 and 114 within the radiating element 116 as indicated in FIG. 1.
In a large percentage of these elements, the hot spot 118 drifts as a function of operating life: first to a position upstream along the positive element 114 and later, after a few thousand hours of operation, well short of the total life of the element, to a position further upstream. For a high performance spectrometer, the optics focus only on the hot spot of the source element, and the drift that occurs would require that the optical alignment be adjusted periodically to maintain optimum performance, which is not acceptable for a user. The alternative is for the spectrometer optics to collect light from a much larger area of the source element, but that makes the effective temperature of the source much lower and therefore reduces the sensitivity of the spectrometer.
As shown in FIG. 1, the source element 100 is made with materials of different resistivity to restrict the hot spot 118 to approximately the middle of the radiating element 116. The problem of hot spot drift occurs when a DC current is applied to the radiating element 116 in order to bring it to a high temperature. The combination of the high current and the high temperature produce a drift of the high resistivity area towards the positive support leg 114. A better approach to solve the foregoing problems is to provide a method that eliminates the position drift of the hot spot of the source.